It has for a long time been a goal of the paper industry to increase the filler content and consequently reduce the fibre content in paper products due to the savings that can be made. The cost of filler is considerably lower than the price of fibres. In addition to cost reductions, an increased amount of fillers may also improve the printability and optical properties of the final paper product. However, using an increased amount of filler may also influence negatively on the paper products. Problems include decrease of the mechanical properties of the paper product. Thus, challenges are found in utilizing more filler in terms of both end product quality and machine runnability. A significant challenge is the loss of strength as the filler disrupts the fibre-fibre bonding network of a sheet by reducing the number of fibres and preventing effective contact of the fibrils. A loss of strength is unfavourable for printing operations as it may cause delamination.
Thus, increasing the binding between fibres and fillers is essential to improve the strength of filled paper.
WO2013/107933 discloses a method for producing paper etc. A stock of fibres is combined with a pretreated filler dispersion comprising mineral filler and cationic pre-treatment agent.
WO2010/125247 discloses a method for preparing aqueous furnish to be used in paper or paper board. The furnish is prepared by addition of filler to a fibre suspension, wherein filler and/or fibres are treated with cationic electrolyte and nanofibrillated cellulose.
Even though different solutions to the problems disclosed above have been presented over the years, there is still a need for new and improved methods and compositions that make it possible to use high contents of filler but without a significant decrease of the strength or other unwanted effects of the final paper product.